Comment 4 for bug 1115177

Ah actually, a bit of googling for the error message showed that this is probably https://bugs.launchpad.net/ubuntu/+source/libdrm/+bug/982889

Essentially, lightdm tries to start up X before the kernel drm driver has finished initializing. Or, perhaps, it is in a race condition with plymouth (disabling splash in the kernel boot command can test this to verify or disprove).

From dmesg:
[ 18.208298] [drm] Initialized nouveau 1.1.0 20120801 for 0000:01:00.0 on minor 0

From Xorg.0.log:
[ 18.207] Initializing built-in extension DRI2

So, it looks to me like DRI2 was initialized before the Nouveau DRM was ready. For some workarounds, see the above bug link.

I'm not sure there is much we can do on the X side, I think we would need to make sure X is not started until the kernel has reported DRM ready (I was under the impression we already had such a check in place, but given that bug 982889 is still open, maybe not?) If you'd like to work towards a real fix for this, can you touch base with Andy Whitcroft?